Motorola MPC823e Reference Manual page 745

Logical "1" represents a chip duration when the transmitting LED is emitting light, while
logical "0" represents a chip duration when the LED is off. Data encoding for transmission is
done LSB first. The 4PPM data encoding defines only the legal encoded payload data
symbols. All other 4-chip combinations are by definition illegal symbols for encoded payload
data. Some of these illegal symbols are used in the definition of the packet envelope
(preamble, start flag, stop flag) because they are unambiguously not data.
16.9.20.3.2 Data Link Layer. The data link layer protocol defines the following packet
format.
The preamble (PA) field is used by the receiver to establish bit synchronization. The PA field
consists of exactly sixteen repeated transmissions of the following stream of symbols.
Figure 16-99. Preamble Field Symbol Format
On the receive side, the PA field does not need to be valid in the received packet. After the
PA field, the receiver starts searching for the start flag (STA) to establish symbol
synchronization. After the start flag is received, the receiver can begin interpreting the data
symbols in the link layer frame. The start flag consists of exactly one transmission of the
following stream of symbols.
0000 1100
MOTOROLA
PA STA
Figure 16-98. High-Speed Packet Format
1000 0000
0000 1100
Figure 16-100. Start Flag Symbol Format
MPC823e REFERENCE MANUAL
Communication Processor Module
LINK LAYER
STO
FRAME
1010 1000
0110 0000 0110 0000
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